Engines may be operated with variable valve timing control to improve engine performance. For example, intake and/or exhaust valve timings may be adjusted (e.g., advanced or retarded) based on engine operating conditions to increase positive or negative valve overlap. Increased positive valve overlap may be used for improving air-fuel mixing and cylinder charge temperature control, for example. As another example, increased negative valve overlap may be used so that flow of air through one or more engine cylinders (e.g., on one engine bank) is substantially reduced while flow of air through other cylinders (e.g., on another bank) is allowed. This enables selective deactivation of the cylinders on the bank with the reduced flow.
However, the inventors herein have identified potential issues with such an approach. As one example, there may be tailpipe emissions and catalyst efficiency issues. Specifically, when the previously deactivated cylinders are reactivated again, additional fuel is required to reactivate the exhaust catalyst of the deactivated bank. This results in a fuel economy penalty. As another example, in cam actuated devices, minor variations in cam timing can affect flow through the deactivated cylinders. In the same way, minor changes in exhaust pressure and intake pressure can result in some net flow between the intake and exhaust manifolds. Any flow from the intake to the exhaust can reduce the efficiency of the exhaust catalyst as well as reduce cylinder deactivation benefits. Overall, engine performance is reduced.
Thus in one example, some of the above issues may be addressed by a method of operating an engine comprising operating a first group of combusting cylinders on a first engine bank to provide a net flow of air and exhaust gas in a first direction while adjusting a valve timing of a second group of non-combusting cylinders on a second engine bank to have substantially less flow in the second bank as compared to the first bank, a direction of the substantially less flow adjusted based on an exhaust air-to-fuel ratio of the second bank. In this way, selective cylinder deactivation benefits may be achieved without degrading the efficiency of an exhaust catalyst on the deactivated engine bank.
For example, an engine may include a first group of cylinders coupled to a first exhaust catalyst on a first engine bank and a second group of cylinders coupled to a second exhaust catalyst on a second engine bank. During selected conditions, such as when an engine load is lower than a threshold, fuel may be injected to, and combusted in, the first group of cylinders. In addition, a valve timing of the first group of cylinders may be adjusted so as to flow air and exhaust gas from an intake manifold towards an exhaust junction through the first exhaust catalyst. At the same time, no fuel may be injected to the second group of cylinders. Instead, a valve timing of the second group of cylinders may be adjusted (e.g., continually adjusted) based on an exhaust air-to-fuel ratio of the second bank so as to have substantially less flow in the second bank as compared to the first bank. Herein, the substantially less flow in the second bank includes a substantially smaller flow in the first direction as compared to flow in the first direction through the first bank during a first condition, and a substantially smaller flow in a second, opposite direction as compared to the flow in the first direction through the first bank during a second condition. By maintaining a substantially smaller amount and rate of flow while continually alternating a direction of the flow through the second bank, a substantially negligible flow (e.g., substantially net zero flow) can be provided through the second group of cylinders.
The valve timing of the second bank may be adjusted based on the exhaust air-to-fuel ratio of the second bank to adjust or alternate the flow. For example, while the first bank is operated at stoichiometry, a leaner than stoichiometry exhaust air-to-fuel ratio at the second bank may be user to infer a small flow of aircharge from the intake manifold to the exhaust manifold. Responsive to the enleanment, the valve timing may be adjusted to reverse flow through the second bank so that a small flow of charge goes from the exhaust manifold to the intake manifold, and the air-to-fuel ratio of the second bank returns to stoichiometry. As another example, a stoichiometric exhaust air-to-fuel ratio at the second bank may be user to infer a small flow of charge from the exhaust manifold to the intake manifold. Responsive to the sensed air-to-fuel ratio, the valve timing may be adjusted to reverse flow through the second bank so that a small flow of charge goes from the intake manifold to the exhaust manifold, and the air-to-fuel ratio of the second bank is enleaned. The continuous alternating of a flow direction of the small flow causes the exhaust air-to-fuel ratio at the second bank to be essentially maintained at slighter leaner than stoichiometry (or slightly leaner than the air-to-fuel ratio of the first bank.
In this way, by reducing flow through a deactivated bank, exhaust catalyst regeneration requirements can be reduced. By maintaining an exhaust air-to-fuel ratio of the deactivated bank slightly leaner than the active bank, substantially zero flow through the deactivated bank can be enabled. By reducing catalyst regeneration requirements during subsequent cylinder reactivation, cylinder deactivation benefits can be achieved without degrading fuel economy and engine performance.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.